Condensed Matter Seminars

A few results from recent neutron and x-ray scattering results will be presented on both CMR and multiferroic systems. For the multiferroics, we have been investigating the magnetic structure of hexagonal HoMnO ₃ as a function of temperature and field [1], which is a commensurate antiferromagnetic (T _N =72 K) ferroelectric (T _C =875 K). Three different chiral symmetries describe the zero field magnetic phases, with strong dielectric anomalies associated with the phase transitions. The spin dynamics are well described by a Heisenberg model in two dimensions. Orthorhombic TbMnO ₃ develops a longitudinally polarized spin density wave state below 41 K, with a change in magnetic structure at 28 K that permits the development of ferroelectricity, while the magnetic structure remains incommensurate [2]. The magnetism is particularly sensitive to Na doping [3]. For the RMn ₂ O ₅ system (R=Tb, Dy, Ho) strong anomalies in the specific heat, thermal expansion, and dielectric constant are a manifestation of the magnetic coupling to the ferroelectricity [4]. Strong magnetoelastic coupling is also found in the triangular antiferromagnetic multiferroic CuFeO ₂ [5]. For the Kagome staircase system Co ₃ V ₂ O ₈ the rich variety of magnetic phases and lock-in transitions is a signature of competing interactions [6], and is quite different from ferroelectric Ni ₃ V ₂ O ₈ . For the CMR systems, we will review recent results for the polaron dynamics in optimally doped La-BaMnO ₃ and La-SrMnO ₃ , and compare these results with La-CaMnO ₃ [7] and the bilayer system [8]. The overall behavior observed in the CMR regime of the manganites is quite similar to that observed in the relaxor ferroelectrics as well as the spin and charge stripes found cuprate oxides, demonstrating a commonality of many of the underlying physical concepts of these perovskite oxides.